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Full text of "Petiolar Glands in the Plum"

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PETIOLAR GLANDS IN THE PLUM 1 

M. J. D orsey and Freeman Weis s 
(WITH PLATES XX, XXl) 

True functional glands are present in the plum in three posi- 
tions: on the leaf serrations, on the leaf base, and on the petiole. 
In the peach, plum, and cherry, the petiolar glands have been 
given a place of considerable taxonomic importance. In the 
course of the fruit breeding work at the Minnesota Agricultural 
Experiment Station, excellent material became available for a 
study of the glands in the plum in certain hybrids and pure forms. 
Since certain questions regarding their variation and morphology 
appeared to be as yet open, the investigation reported herein was 
begun. 

In a historical review of the taxonomic use of the petiolar 
glands in the stone fruits, Gregory (3) showed that the earlier 
writers had ignored these structures; while later pomologists had 
made use of them in distinguishing major groups, as in the peach. 
Other writers, however, questioned the taxonomic value of glands, 
because of the variation observed in number, shape, and position. 
From an extensive study of the leading varieties of the peach, 
Gregory concluded that on typical shoots the glands were con- 
stant, and that in many cases their shape could serve to separate 
groups of varieties. He arranged the better known peach varieties 
under three types of glands, reniform, globose, and indistinctive, 
but pointed out that mixed and transitional types occur. 

Hedrick and others (4) record the gland condition on the 
petiole and leaf serrations in the descriptions of the principal 
varieties of plums in New York. Similar data have been brought 
together for cherries (Hedrick et al. 5) and peaches (Hedrick 
et al. 6). In the latter work the statement is made that "no one 
familiar with any considerable number of varieties of peaches 

1 Published with the approval of the Director as Paper no. 160 of the Journal 
Series of the Minnesota Agricultural Experiment Station. The writers acknowledge 
their indebtedness to Dr. C. O. Rosendahl for criticism and suggestions, and to 
Ernest Dorsey and James Gray for assistance in collecting and classifying material. 

391] [Botanical Gazette, vol. 69 



392 BOTANICAL GAZETTE [may 

would attach very great importance to glands in a system of 
classification." 

On the whole, the tendency of later writers has been to attach 
less significance to glands in classification than has been done by 
earlier writers. In technical fruit descriptions, or in systematic 
classifications, it is evident that the value of a character as a 
distinguishing feature between forms depends largely upon its 
constancy of expression. Consequently, a statistical analysis was 
undertaken with the object of determining the number and dis- 
position of glands in certain species and hybrids available. 

Material 

Data were first collected in 1914 in the F x generation of crosses 
between Burbank (Prunus triflora) and Wolf (P. americana), and 
Abundance (P. triflora) and Wolf. The gland condition was sub- 
sequently (August 1 91 6) obtained in an additional number of 
species and interspecific hybrids. Single trees in each case of as 
nearly uniform age and size as possible were selected, and 400 
leaves, on all trees which bore this number, were taken at random 
from vigorous i-year shoots. By following this method of collec- 
tion consistently on trees under fairly uniform growth conditions, 
the data obtained for the different forms are as nearly comparable 
as can be obtained under field culture. 

There are a number of factors which influence gland develop- 
ment. In general it may be stated that those conditions which 
produce vigorous vegetative growth favor gland development, 
since on old trees or on trees subjected to unfavorable growth 
conditions, the petiolar glands become much reduced, some- 
times even disappearing, although normally present in the varie- 
ties. On the other hand, position has an influence on glandular 
development. Leaves borne at the basal position on terminal 
growth, on fruit spurs or thorns and also in flower buds, typically 
bear no glands at all or have them less well developed than leaves 
borne at other points. 

The arrangement of the glands (that is, whether opposite or 
alternate on the petiole or leaf) was not recorded. Glands occur 
both in pairs and alternately, near together or widely separated, 



1920] 



DORSEY &• WEISS— PETIOLAR GLANDS 



393 



but since they vary independently on either side of the petiole, 
their relative position appears to be only incidental. 

Variation in gland position and number 

In horticultural literature, glands have been described with 
respect to color, type or shape, size, number, and position. In 

TABLE I 

Selected instances illustrating method of recording data and showing vari- 
ability of glands (a) on different leaves within a variety, (b) with 
reference to position on petiole or leaf base, (c) with reference to 

DIFFERENT VARIETIES, AND (d) WITHIN SAME VARIETY DURING DIFFERENT SEASONS. 



NO. BORNE ON 


Abundance XWolf 

NO. 35 


BURBANK 


BurbankXWolf no. g 


Peti- 
ole 


Leaf 


1914 


1917 


IOI4 


1917 


1014 


1017 


O 


O 


15 

27 
28 


38 
35 
37 
1 
40 

33 
16 


IO 
16 

32 


45 
63 
45 


44 
25 
28 


52 
44 
45 


O 


I 


O 


2 


O 


3 


I 


O 


34 
29 

1 


6 
8 
9 


8 
29 

25 

1 

65 
47 
18 

1 
20 
17 


25 

63 

9 


33 

48 

6 


I 


I 


I 


2 


I 


3 




2 


O 


219 

39 

4 


188 
10 


67 
45 
33 


141 

49 

1 


161 


2 


I 


5 

1 


2 


2 


2 


3 






3 
3 
3 

4 
4 
4 
5 
5 
5 
5 
6 


O 


11 
3 


2 


55 
33 
12 

33 
24 

9 

2 

3 
1 


8 
6 


4 


I 


2 






O 






7 
3 

1 
2 
1 
1 

1 






I 


1 




1 




2 

















I 










2 










3 










O 






2 




















Glands on 

petiole 

Glands on leaf . 


604 
143 


491 

187 


919 
321 


505 
349 


525 
220 


437 
202 



the plum the globose form is the prevailing type, and the true 
reniform type is found so seldom that little attention has been 
given to shape. The color of the mature glands in the plum is 
dark brown; and since these studies of number and position were 
made on mature leaves, color characters were also not recorded. 



394 BOTANICAL GAZETTE [may 

Data taken as to position and number were arranged in the 
form illustrated in table I, in which each leaf is classified with 
respect to the position and number of its glands. For instance, 
in Burbank 67 leaves bore two glands on the petiole and none on 
the leaf in 1914, and in 1917, 65 leaves fell in this class. A number 
of other varieties could have been included, but these were selected 
as typical of the great variability encountered. 

Table I shows that in number and position glands are extremely 
variable on different leaves within a variety, but that the range 
of variability is fairly typical for each variety. The number of 
glands borne on the petiole is greater than the number borne on 
the leaf base, and while the number borne in each position is 
considerably different from season to season, yet the grouping 
opposite each class is quite similar in each variety in spite of the 
fact that the 191 7 data were taken from different trees, but of 
the same clones, from those of 1914. Taking Burbank again as an 
illustration of variability, it will be seen that some leaves have 
no glands on either the petiole or leaf, while others bear as many 
as five on the petiole and three on the leaf. If observations as to 
gland condition made on a few leaves or herbarium specimens are 
considered from the standpoint of the variation shown, it will be 
evident that some caution must be exercised in classifying the 
gland condition. 

Referring to the variability of glands within the species, it 
will be seen that a similar condition is found to that shown within 
varieties. A summary of the position and number of glands in 
all the species investigated is presented in table II, in which the 
gland condition is given for a total of 3477 leaves. 

Four points are of interest in table II: (1) without exception 
there are more glands borne on the petiole than on the leaf base; 
(2) when there is one gland present it may be borne either on the 
leaf base or on the petiole; (3) when two glands are present, the 
larger number is without exception borne on the petiole; and (4) 
when more than two glands are present, without exception a 
strikingly larger number occur on the petiole. 

For the convenience of the reader the data presented in table 
II, with the addition of data from certain interspecific hybrids, 



1920] 



DORSEY 6* WEISS— PETIOLAR GLANDS 



395 



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396 



BOTANICAL GAZETTE 



[may 



are rearranged in table III, to emphasize the modal class for gland 
number on the petiole and on the leaf base. It will be seen in all 
the combinations summarized therein that the mode for gland 
number on the petiole (with only three exceptions) is two, and 
that on the leaf base it is zero. In other words, the 2-0 condition 
shown in figs. 1 and 11 is typical in the plum. Thus it appears 
that the glands in the plum are too variable, both in position and 
number, for accurate statements concerning their disposition on 

TABLE III 

Summary or gland condition in all forms studied, showing modal class of glands 

BORNE ON PETIOLE AND LEAF BASE. 



Species or cross 



ww§S 

BWhS 



Total 

LEAF 
NUMBER 



Glands borne on petiole 



Number Mode Mean 



Glands borne on leaf 



Number Mode Mean 



Abundance (P. trifiora) 

Burbank (P. trifiora) 

Wolf (P. americana mollis) . . . 
Abundance X Wolf crosses. . . 

Burbank X Wolf crosses 

P. americana 

P. Besseyi (Sand cherry) 

P. Besseyi crosses 

P. BesseyiXP. hortulana 

mineri (Compass) 

Compass crosses 

P. cerasus 

P. domestica (Shippers' pride) 
P. domesticaXP. americana. . 

P. pennsylvanica 

P. Simoni 



1 

22 

3 

1 
6 



400 

400 

400 

15200 

8000 

877 
400 
800 

400 

1437 
400 
400 
200 
400 
200 



409 

9i3 

607 

21956 

10406 

350 

407 

3359 

428 
2019 

707 
35 
89 

554 

438 



1.023 
2.285 
1. 5i8 
1.440 
1.300 
0.406 
1. 018 
1.866 

1.070 
1405 
1.768 
0.088 
o.455 
1-385 
2.019 



333 

39 

9125 

5028 

212 

107 

788 

212 

514 
242 
287 
112 
8 
33 



o.473 
0.833 
0.010 
0.600 

0.628 

0.242 

0.268 

o 438 



0.530 
0.357 
0.605 
0.718 
0.560 
0.020 

0.165 



any other basis than that of a statistical analysis. Other char- 
acteristics of glands, however, may be of distinctive value, and 
gland disposition may be sufficiently different and characteristic 
for a variety or species to be of taxonomic value. The outstanding 
feature of the data on the disposition of glands is that the mode 
for gland number on the petiole, with three exceptions, falls on two, 
and that the mode for the number borne on the leaf base in every 
instance is zero. This condition obtains notwithstanding the 
great diversity in the forms under investigation, and even in the 
three exceptions to a modal class of two on the petiole, the mode 



i 9 2o] DORSEY & WEISS— PETIOLAR GLANDS 397 

was zero. This is significant in view of the fact that these three 
forms, P. atnericana, P. domestica (Shippers' pride), and P. domes- 
ticaXP. arnericana, showed the greatest suppression of the glands 
of any of the forms included in this investigation. This condition 
will be given even greater emphasis in view of the connection found 
to exist between glands and the vascular system of the leaf. 

Connection of petiolar glands with vascular system 

With the status of the glands as to number and position shown 
by the statistical analysis in mind, it now remains to be seen 
whether or not there is a basic cause for the predominance of the 
2-0 frequency. 

The leaf trace in the plum has three bundles of conducting 
tissue at its departure from the vascular cylinder of the stem, 
which cause three gaps in the woody cylinder. The central bundle 
extends up the petiole, through the blade, and branches succes- 
sively at the large lateral veins. The outer bundles give rise to 
strands which run along the upper side of the petiole, forming 
pronounced ridges on either side. These strands run directly to the 
petiolar glands, or to the large ones borne on the leaf base, and 
terminate there. The term petiolar gland, therefore, will be used 
in this connection to include both. The lateral strands are shown 
in fig. 30, which was drawn from a young leaf, and they are equally 
conspicuous whether the glands are borne on the leaf base or some 
distance down on the petiole. Where there is more than one 
gland on either side of the petiole, branches from the lateral strands 
connect with them. 

On the other hand, the glands of the leaf serrations have a 
distinctly different vascular connection. Instead of being con- 
nected by branches with the lateral strands, glands in this position 
have their vascular connections with the central bundle through 
the lateral veins. It appears, therefore, that on the basis of 
differences in their vascular connections the petiolar glands and 
the glands borne on the serrations can be placed in two distinct 
classes. Other considerations also support this view. The petiolar 
glands are much larger than those on the serrations and may 
show differences in shape, such as the reniform or necked types, 



398 BOTANICAL GAZETTE [may 

which would readily differentiate them. Also under orchard con- 
ditions the glands on the petiole may be active much later in the 
season. That there is no relation between the glands in the two 
positions is further shown by the fact that in some species, as in 
P. americana, glands are typically absent on the serrations but 
present on the petiole or leaf base. 

Glands which could not readily be classified as belonging to 
either the leaf serrations or the petiole were not numerous, con- 
sequently error from this cause has not entered to any appreciable 
extent into the statistical classification. There was also little 
difficulty in determining whether glands were borne on the petiole 
or leaf base, since in most cases there was no leaf tissue between 
the glands and the base of the blade. When leaf tissue was so 
present they were classed as being borne on the leaf base. 

The question now arises as to whether the structure of glands 
on the petiole is similar to that on the leaf base. Gregory (3) 
showed that glands borne on the petiole and leaf base in the peach 
were true glands, the upper part being composed of long rec- 
tangular cells rich in cytoplasm, and with large nuclei, while the 
central part is made up of parenchyma cells characteristic of 
glandular tissue, into which extend ramifications of conductive 
tissue. The structure of the glands in the plum borne on both 
the petiole and serrations has been examined on leaves just emer- 
ging from the bud, and on mature leaves with vigorous active 
glands, and is found to be similar to that reported by Gregory in 
the peach. 

The similarity in structure between the glands borne on the 
petiole and on the serrations led Gregory to suggest that the 
former arose from the latter. Such an origin would imply that both 
are of the same rank, and that the tissue of the leaf blade is more or 
less indeterminate with that of the petiole, and would be in accord 
with the condition of the petiolar glands in some species of willow, 
notably Salix lucida (fig. 10), in which the glands are minute, 
numerous, and crowded together at the leaf base, suggesting a 
proliferation of leaf tissue along the petiole. 

From these considerations it appears that there is justification 
for regarding the glands of the petiole, and the larger ones borne 



1920] DORSEY & WEISS— PETIOLAR GLANDS 399 

on the leaf base, as of the same structure but of different rank 
from those borne on the serrations. This is in keeping with the 
evidence presented in the statistical analysis in which the number 
two figured so prominently, that is, two glands on the petiole and 
none on the leaf base, or one in either position, or less frequently, 
two on the leaf base. The points emphasized are of significance 
from the standpoint of phylogeny, and will be given greater em- 
phasis from that standpoint. 

Phylogeny of leaf as indicated by glands, vascular system, 
stipules, and abscission layers 

There are structures other than the glands whch are significant 
from the standpoint of the ancestral type of leaf in Prunus and 
related genera. Cook (1) called attention to a joint in the leaves 
of Amygdalaceae "just above the insertion of the stipules/' and 
states that the "basal section of the leaf below the joint" has a 
separate abscission from that of the leaf proper. This "basal sec- 
tion" is regarded as belonging to the leaf on the basis of the 
attachment of the stipules to it, although the stipules themselves 
absciss early, and hence "the persistence of the base of the leaf" 
has been overlooked. Goebel (2) characterizes stipules as "ap- 
pendages arising at the insertion of a leaf, attached either wholly 
to the petiole or to the stem, or to both." Sinnott and Bailey 
(12) regard the stipules as arising through the stimulus of the 
growth of the lateral leaf trace, and although they are morphologi- 
cally integral parts of the leaf, in some exstipulate families having 
trilacunar nodes, they are represented by mere swellings opposite 
the leaf trace. In some stipulate genera with opposite leaves, as 
Viburnum, the stipules arise midway between the insertion of the 
petioles, apparently directly from the stem. Cook also states that 
in Texas this basal section in the peach may remain alive for a 
year or two and then wither away; while in Maryland it lives 
through the winter and separates in the spring, leaving a fresh 
green scar. 

In most of the plums studied in this investigation, this structure 
does not separate at all, but may be clearly observed (the petiole 
scar with its three bundle scars, and the two stipule scars at its 



400 BOTANICAL GAZETTE [may 

sides) on 3-year or even 4-year-old wood. Some vestige of it 
often remains a year or two longer, but usually after this time it 
begins to slough off. 

It was not until some young trees of Amygdalus Davidiana, 
which were making an extremely rapid spring growth, were 
examined that the separation which Cook described was found. 
Ordinarily it requires some effort to remove the dry scalelike 
remnant left after the fall of the leaf, and usually there is more 
or less tearing of the bark; but in the case of Amygdalus Davidiana 
this structure separated easily and clearly when started with a 
knife point. In many cases after the rapid spring growth it 
cracked and separated without any outside stimulus, and later a 
large proportion fell as a result of further enlargement at the 
node. The condition is represented in tig. 5, which shows the 
separating scale, and fig. 31, which shows its structure in greater 
detail. 

Examination of rapidly growing shoots of other Prunus species 
showed that a similar separation may take place, especially in 
some of the P. BesseyiXP. triflora hybrids, although in no case 
quite so sharply as in Amygdalus. At least a partial explanation 
of the fact that the separation of the scar scale occurs in some 
species of Prunus and not in others, can be made on the basis of 
the character of the swelling of the node below the leaf insertion. 
When it is straight in outline, it makes an acute angle with the 
line of the shoot and terminates in the leaf scar along a narrow 
ridge, as in Amygdalus (fig. 5). The rapid growth in spring of 
tissue underneath forces off the dead scale at the apex. If, how- 
ever, the node is swollen, with a rounded profile, and the leaf scar 
is well buttressed below, as in the extreme type shown in fig. 4, 
the scale does not separate, and cannot be removed without tear- 
ing the bark. In either case, whether separation is immediate 
or delayed, it is only the shedding of dead tissue, just as bark is 
shed, and is done without the aid of a definite abscission zone 
(Loyd 9), and consequently is not true abscission. 

A search for corroborative evidence as to the nature of this 
structure showed that in a number of genera it is not uncommon 
for clean cut separation of a scar scale to occur when rapid growth 



i 9 2o] DORSEY & WEISS— PETIOLAR GLANDS 401 

begins. Some of the species which show the separation of the old 
leaf scar more clearly even than Amygdalus are Shepherdia argentea, 
Comus stolonifera, Tilia americana, Rhamnus cathartica, and Celtis 
occidentalism the first two of which are illustrated in figs. 16 and 
17. All of these have simple leaves and lack petiolar glands, and 
Comus and Shepherdia are exstipulate and have entire leaves, 
which, according to Sinnott and Bailey (ii, 12), is an advanced 
state of node and leaf morphology. Hence we must regard the 
structure to which the stipules and petiole in Prunus are articu- 
lated, not as an additional foliar element, but as an outgrowth 
from the stem; and therefore abscission of this kind does not 
necessarily indicate an additional foliar element. 

The manner of insertion and abscission of the stipules in the 
plum furnishes additional evidence that the structure described 
by Cook as a persistent leaf base is in reality a part of the stem, 
and that the leaves of Amygdalaceae cannot therefore be jointed. 

In some Rosaceous genera, as Potentilla and Rosa, the stipules 
are adnate to the petiole, forming a somewhat sheathing base, and 
fall with the leaf. In others, as Pyrus and Prunus, the stipules 
are separate, or nearly separate, from the petiole, and absciss soon 
after the buds unfold. In Prunus the stipules usually drop long 
before the leaf, but occasionally they persist throughout the grow- 
ing season, and even over winter, in vigorous, late growing branches 
in which cold weather has stopped further growth. The point 
of normal abscission is illustrated in fig. 32 (P. hortulana rnineri). 
The stipules, like the petiole, separate at a definite abscission 
layer at their base (figs. 1, 2). As Cook pointed out, the joint of 
the latter lies above the stipules, that is, distally to them, although 
since the abscission lines of the petiole and the stipules form a 
sort of crescent with the points upward, a face view of the stem 
shows the stipule scars above that of the petiole (figs. 31, 32). 

The typical leaves of many stipulate genera of the Rosaceae 
have stipules adnate to the petiole, forming a more or less sheathing 
base. This condition is also to be found in the bud scales and the 
scales transitional to leaves in Prunus. Morphologists regard bud 
scales as relatively primitive in structure, since they have not 
specialized to serve such varied functions as the leaves themselves. 



402 BOTANICAL GAZETTE [may 

Accordingly, the bud scales of Prunus may be taken as an index 
of the ancestral leaf type. A series of bud scales and scale leaves 
grading into true leaves is shown in figs. 18-29, which were taken 
from a young shoot of the Compass cherry {P. BesseyiXP '. hor- 
tulana mineri). 

It may clearly be seen that in the outer scales the stipules are 
represented by blunt lobes, as large as the central lobe which 
represents the leaf blade. The inner scales show progressive 
reduction in size of the lateral lobes and increase of the middle 
one, accompanied by differentiation into stipules and lamina. It 
will be noted, also, that there is progressive splitting of the stipules 
from the petiole. In the mature leaf of the plum this splitting 
has progressed to the base of the petiole, so that leaf and stipules 
have separate abscission, but from one originally continuous 
abscission layer. Prunus avium, as represented by the Dyehouse 
cherry, is at a somewhat intermediate stage. In the leaves near 
the base of a shoot, the stipules are clearly adherent to the petiole, 
while in the upper leaves they are separate, as in the plum. In 
all these transitional bud scales separation from the axis is clearly 
below the stipules, and there is no evidence of an abscission layer 
above them cutting off the middle lobe or leaf blade (fig. 2) ; hence 
the stipules in the plum must be regarded morphologically as 
integral with the leaf base, although the course of development 
separates them from the petiole in the mature leaf. 

It remains to interpret the structure of the petiolar glands in 
relation to a more primitive type of leaf. Their organization and 
their occasional proliferation into leaflike outgrowths indicate that 
they are reduced structures. This change from a glandular to a 
foliar structure was noted by Cook (i), who described the occur- 
rence of " small oblong or spatulate leafy organs on the upper 
part of the petiole, taking the place of the nectaries" in certain 
varieties of apricots. A similar transformation has been observed 
in Crataegus, as well as a number of species of Prunus, and in some 
of the apricot hybrids at the Minnesota Experiment Station it is 
of almost regular occurrence on vigorous shoots of young trees. 
Some of the variations in this transformation are illustrated in 
figs. n-15. The view that the leaves of the Amygdalaceae are 



i92o] DORSEY fir WEISS— PET IOLAR GLANDS 403 

jointed, led Cook to conclude that the ancestral leaf type of this 
group was compound, the nectaries representing " rudiments of 
divisions of compound leaves." It should be pointed out, however, 
that if the modern plum leaf represents the terminal leaflet of an 
originally compound leaf, the glands, representing reduced lateral 
leaflets, should be found below the joint by which the terminal 
leaflet is articulated to the rachis. It is obvious that being situated 
on the petiole above the joint, they cannot represent lateral leaf- 
lets of an ancestral pinnate form. Microscopic sections of the 
petiole at the leaf base do not show an abscission layer subtending 
the blade, and the presence of one is not indicated by the normal 
manner of shedding the leaves. It may be safely concluded, there- 
fore, that the leaf blade of the plum is not articulated to the 
petiole, as in Citrus, Herberts, or Trifolium. Sometimes, however, 
if there has been late growth in the fall and the leaves are immature 
when the first frosts occur, the leaf axis may be broken anywhere 
from the middle of the blade to the base of the petiole, although 
most frequently at the juncture of the blade and petiole. In this 
case the twig enters the winter with a part or all of the petiole 
adhering at each node, which, however, usually breaks off at the 
point of normal abscission by spring. 

Glands, however, may represent divisions of an alternate 
divided or pinnatifid leaf, such as Fragaria, Potentilla, and other 
genera of the Rosaceae possess. The frequency of the occurrence 
of two glands suggests a ternate leaf such as that of Fragaria. 
The presence of additional glands may be accounted for on the 
basis of branching of the lateral bundles, or by the common transi- 
tion from ternate to quinquefoliate or pinnate leaves (Lewis 
7,8). Some pinnate-leaved forms of Potentilla show reduction of 
the lower divisions to smaller structures than are the leafy out- 
growths in the position of glands in the apricot. Furthermore, 
the lower divisions are frequently alternate instead of paired, as 
is also the case with the petiolar glands in Prunus. 

It will be recalled that in the plum the two lateral bundles 
which connect with the glands terminate there, and do not con- 
tribute extensively to the vascular system of the blade. In 
ternate leaves which have three strands, the outer two provide the 



404 BOTANICAL GAZETTE [may 

vascular system of the lateral leaflets, and this is also frequently 
the case when there are numerous lateral leaflets, as in pinnate 
forms of Potentilla. 

On foliar evidence the connection of the stone fruits, whether 
regarded as a tribe of the Rosaceae or as a separate family, with 
the Potentilleae is quite direct. The nodal anatomy is the same 
(Sinnott and Bailey 12) and the steps in leaf evolution appear 
to be: (1) reduction of the lateral leaflets of a ternate or pinnate- 
divided leaf to petiolar glands; and (2) splitting of persistent 
adnate stipules from the petiole. In this series Prunus avium, 
with an extensive development of glands, both in number and 
size, or the apricot with a frequent reversion to leafy structures in 
place of glands, would stand below Prunus americana with its 
almost glandless petioles. Gregory (3) regarded the globose 
gland as more primitive than the reniform, since when normally 
glandless leaves produced glands, they were always of a globose 
type. If, however, the trend is toward reduction of glands, the 
glandless petiole would be the highest type, and the globose con- 
dition transitional between it and the reniform. 

It has been customary to trace the connection of the drupes 
with the more primitive Rosaceae through Spiraea (Rydberg 10). 
Considered on the basis of floral evidence alone, this seems a logical 
sequence, Spiraea being intermediate in form of receptacle and 
number of carpels between Potentilla and Prunus. Sinnott and 
Bailey (12) have shown, however, that Spiraea is exstipulate 
and possesses a unilacunar node, while all species of Prunus have 
stipules and a trilacunar node. It would appear that forms with 
stipules could not be derived from forms which lack them. The 
relationship of the stone fruits to the true Rosaceae is probably 
more direct, and on the basis of anatomy there is less reason for 
separating them as a distinct family than for considering them a 
well defined and specialized tribe. 

Summary 

Examination of over 30,000 leaves belonging to 15 species and 
interspecific hybrids of the plum shows that two glands typically 
occur on the petiole, or less frequently on the leaf base. 



1920] DORSEY fir WEISS— PETIOLAR GLANDS 405 

On the basis of vascular connections the glands on the petiole 
or leaf base are of a different order of structure from those on the 
leaf serrations. 

The stipules in the plum are morphologically integral with the 
leaf base, and separate from the stem by a common abscission 
layer. A portion of the node bearing the leaf and stipule scars 
is subsequently shed in some species of Prunus as in other woody 
genera; but the portion thus shed is not an additional foliar 
element. 

On the basis of nodal anatomy and the presence of reduced 
structures, the ancestral type of leaf in the plum is considered to 
be a ternate lobed or divided simple leaf, the petiole glands repre- 
senting the suppressed lateral members. In floral structure and 
nodal anatomy Prunus and related genera form a logical series 
with the Potentilleae, and should be considered as a specialized 
tribe of the Rosaceae. 

Agricultural Experiment Station 
University of Minnesota 

LITERATURE CITED 

1. Cook, O. F., Jointed leaves of Amygdalus. Jour. Wash. Acad. Sci. 
2:218-220. 1912. 

2. Goebel, K., Organography of plants. Eng. trans, by Balfour, Oxford. 
1905. pp. 359-376. 

3. Gregory, C. T., The taxonomic value and structure of the peach leaf 
glands. N.Y. Agric. Exp. Sta. Bull. 365. pp. 183-222. figs. 2. 191 5. 

4. Hedrick, U. P., et al., The plums of New York. Report N.Y. Agric. 
Exp. Sta. 1910, vol. 2. 1910. 

5. , The cherries of New York. Report N.Y. Agric. Exp. Sta. 1914, 

vol. 2. 1914. 

6. , The peaches of New York. Report N.Y. Agric. Exp. Sta. 191 6, 

vol. 2. 1916. 

7. Lewis, F. T., The development of pinnate leaves. Amer. Nat. 41:431- 
441. figs. 4. 1907. 

8. , A further study of leaf development. Amer. Nat. 41:701-709. 

figs. 15. 1907. 

9. Lloyd, F. E., Abscission in Mirabilis Jalapa. Box. Gaz. 61:213-230. 
pi. 1. 1916. 

10. Rydberg, P. A., Monograph of the North American Potentilleae. Mem. 
Dept. Bot. Columbia University. 



406 BOTANICAL GAZETTE [may 

ii. Sinnott, E. W., The anatomy of the node as an aid to classification of 
angiosperms. Amer. Jour. Bot. 1:303-322. pi. 4. 1914. 

12. Sinnott, E. W., and Bailey, I. W., Investigations on the phylogeny of 
the angiosperms. 3. Nodal anatomy and the morphology of stipules, 
Amer. Jour. Bot. 1:441-453. pi. 1. 1914. 

EXPLANATION OF PLATES XX, XXI 

PLATE XX 

Fig. i . — Section through node showing abscission zone by means of which 
the leaf is shed; a, only abscission zone present. 

Fig. 2. — Showing continuous abscission zone (a) for stipule and petiole at 
early stage of growth. 

Fig. 3. — Dark line at base of stipule showing point of abscission; note 
that it is distal to attachment of petiole. 

Fig. 4. — Instance of extreme swelling in node below point of abscission 
of leaf; in such instances scar scale is not shed. 

Fig. 5. — Enlarged view of shedding of scar-scale (a) in Amygdalus David- 
iana as result of early spring growth; shedding of this type occurs only when 
contour of node is relatively flat. 

Figs. 6-9. — Variation in gland position and number; types illustrated 
are 2-0 (2 on petiole and o on leaf base), 1-0, 1-1, and 0-1. 

PLATE XXI 

Fig. 10. — Leaf of Salix lucida showing glands similar to those of leaf 
serrations crowded at base. 

Figs. 11-15. — Types of proliferation of petiolar glands into leafy out- 
growths. 

Fig. 16. — Node of Shepherdia argentea showing shedding of scar scale 
similar to Amygdalus. 

Fig. 17. — A node of Comus stolonifera showing same as 16. 

Figs. 18-28. — Bud scales and basal leaves from young shoot of Dye- 
house cherry, presenting series in separation of stipules from leaf blade. 

Fig. 29. — Enlarged basal leaf illustrating venation of stipules and leaf 
blade ; note that veins of stipules arise as branches of lateral strands. 

Fig. 30. — Petiole showing prominence of lateral bundles in young leaf. 

Fig. 31. — Shedding of dead scar scale in Amygdalus Davidiana from which 
stipules and petiole abscissed previous season. 

Fig. 32. — Node of young plum stem showing relation of line of abscission 
in stipule and petiole. 



BOTANICAL GAZETTE, LXIX 



PLATE XX 




DORSEY & WEISS on GLANDS 



BOTANICAL GAZETTE, LXIX 



PLATE XXI 





FM *3 



x3 *- x& 
DORSEY & WEISS on GLANDS